Electrical energy storage is needed in many applications, such as electric/hybrid vehicles, portable electronic devices, and power systems. Traditionally, batteries of various kinds have been used for most applications. However, use of batteries in many new applications is complicated by their limited charge cycling ability, relatively slow discharge rates, and the toxicity of the chemicals incorporated into the batteries.
In recent years, electrochemical double layer capacitors (EDLCs, also referred to as ultracapacitors or supercapacitors) have emerged as an alternative to batteries in applications that require high power and long shelf and cycle life. Rather than two individual plates separated by an intervening substance, these capacitors use “plates” that are in fact two layers of the same substrate, the so-called “electrical double layer.” As the name suggests, energy storage in an EDLC is achieved by separating and storing electrical charges in the electrochemical double layer at the interface between a solid surface and an electrolyte. The electrical properties of the electrical double layer result in the effective separation of charge despite the vanishingly thin (on the order of nanometers) physical separation of the layers. The lack of need for a bulky layer of dielectric permits the packing of “plates” with much larger surface area into a given size, resulting in their extraordinarily high capacitances in practical sized packages.
Activated carbon (or active carbon) is the most widely used material in EDLCs thanks to its very large surface area, good electrical and ionic conductivity, excellent chemical stability, and low cost. Alkali activation is one process for forming activated carbon. It relies on the carbonization of a carbonaceous precursor compound in an inert atmosphere at high temperatures, followed by chemical activation, typically using KOH or NaOH. One major disadvantage of alkali activation is that alkali metal can be produced as a reaction by-product, which can volatize and condense out in colder regions of the process equipment. This poses a significant safety hazard for large-scale production. In addition, it causes severe corrosion to the process equipment because of the volatile nature of alkali metals. Therefore, the alkali metal generated in the activation process must be “treated” before being exposed to ambient environment. Previously, treatment has been done by introducing water vapor, CO2 or both to the equipment after the material has been cooled, thus converting the alkali metal to corresponding hydroxide and carbonate, which can then be safely discharged. This process has two disadvantages: first, extra process cycle time (for a batch process) or extra equipment such as a cooling chamber (for a continuous process) are required for the treatment and add to the process cost; and second, since the alkali metal is not treated until the end of the process cycle, safety issues pose a serious concern. If there are any cold spots in the equipment, the alkali metal tends to deposit and accumulate in those spots. As the deposit grows thicker, the amount of time required post-activation for water vapor or CO2 to penetrate the deposit becomes longer. This increases the risk that an accident may occur during the activation process and requires longer process cycle times. In this disclosure, we describe a novel alkali activation process that addresses the above issues.